I can see that some sites have a estimate cost of postage -
Since postage is based in most parts on weight and the zip code then - You could have a little place on the bottom of an item that says estimate cost or postage before the client types in their address.
* the reason is often for sending an item overseas- eg here is the king of online selling in australia.
Women love this store - http://www.petersofkensington.com.au/Public/Default.aspx - just to give an example.